Conventionally, OCT has been used to understand the internal information, or specifically the differential structure represented by backward scattering, reflectance distribution, and refractive index distribution, of the target (sample), in a non-destructive manner at high resolution.
“Optical coherence tomography” (OCT) is a non-destructive tomography technique used in the field of medicine, etc. (refer to Patent Literature 1). One advantage of OCT is that, because it uses light as a measurement probe, OCT can be used to measure the reflectance distribution, refractive index distribution, spectral information, and polarization information (birefringence distribution), etc., of the measuring target (sample).
The basic OCT 43 is built on the Michelson interferometer, whose principles are explained in FIG. 5. Light emitted from the light source 44 is parallelized by the collimator lens 45, and then split into reference light and object light by the beam splitter 46. The object light is collected onto the measuring target 48 by the object lens 47 in the object arm (sample arm), upon which it is scattered/reflected and then returns to the object lens 47 and beam splitter 46.
On the other hand, the reference light transmits through the object lens 49 in the reference arm, upon which it is reflected by the reference mirror 50 and returns to the beam splitter 46 through the object lens 49. The reference light, thus returning to the beam splitter 46, enters the collecting lens 51 together with the object light, and both lights are collected onto the photo-detector 52 (photo-diode, etc.).
For the OCT light source 44, a light source of low temporal coherence light (where it is extremely difficult for lights emitted from the light source at different times to interfere with each other) is used. With the Michelson interferometer using a light source of low temporal coherence light, an interference signal shows up only when the distance from the reference arm is roughly equivalent to the distance from the object arm. As a result, measuring the intensities of interference signals with the photo-detector 52 by changing the optical path difference (τ) between the reference arm and object arm gives relationship between the interference signal and the optical path difference (interferogram).
The shape of this interferogram represents the reflectance distribution of the measuring target 48 in the depth direction, where one-dimensional scan in the axial direction reveals the structure of the measuring target 48 in the depth direction. As described above, the OCT 43 uses optical path length scan to measure the structure of the measuring target 48 in the depth direction.
Such axial-direction (A direction) scan can be combined with mechanical scan in the lateral direction (B direction) (B scan) to perform two-dimensional scan in order to obtain a two-dimensional cross-section image of the measuring target. For the scanner that performs this lateral-direction scan, one constituted in such a way that the measuring target is moved directly, or that the measuring target is fixed and the object lens is shifted, or that the measuring target and object lens are fixed and the angle of the galvano-mirror placed near the pupillary surface of the object lens is rotated, is used, among others.
Extensions of the basic OCT mentioned above include the swept source OCT (SS-OCT) that scans the wavelengths of the light source to obtain spectral interference signals, and the spectral domain OCT that uses a spectrometer to obtain spectral signals. The latter is divided into the Fourier domain OCT (FD-OCT, refer to Patent Literature 2) and the PS-OCT (refer to Patent Literature 3).
With the swept source OCT, the wavelength of the light source is changed with a high-speed wavelength scanning laser and the light source scan signals obtained simultaneously with the spectral signals are used to rearrange the interference signals, to which signal processing is applied, to obtain a three-dimensional optical tomography image. It should be noted that, with the swept scan OCT, a monochromator can be used as the means for changing the wavelength of the light source.
The Fourier domain OCT is characterized in that a spectrometer is used to obtain a wavelength spectrum of the reflected lights from the measuring target and then this spectral intensity distribution is Fourier-converted to take out signals in the real space (OCT signal space), and with this Fourier domain OCT, the cross-section structure of the measuring target can be measured by scanning in the X-axis direction, without having to scan in the depth direction.
The PS-OCT is an optical coherence tomograph that can successively modulate the polarization state of the beam that has been linearly polarized concurrently with B scan, to understand the polarization information of the sample (measuring target) and thereby measure a more detailed structure, and the anisotropy of the refractive index, of the sample.
To be more specific, the PS-OCT uses a spectrometer to obtain a wavelength spectrum of the reflected lights from the measuring target, just like the Fourier domain OCT does, where the incident light and reference light are each passed through a ½ wave plate, ¼ wave plate, etc., to be polarized horizontally linearly, vertically linearly, linearly at 45 degrees, or circularly, while the reflected light from the measuring target and reference light are superimposed and passed through a ½ wave plate, ¼ wave plate, etc., after which the horizontal polarization components alone, for example, are entered into the spectrometer to cause interference, and the object light components in specific polarization states are taken out and Fourier-converted. This PS-OCT does not require depth-wise scan, either.